The present invention relates to a shield to be applied to a mechanical device for measuring the pressure within a human eye which, at the same time, improves the ability of the operator to make accurate pressure determinations.
The internal pressure in the human eye is usually determined through the use of a tonometer. A common type of tonometer is the applanation tonometer which determines internal pressure as a function of corneal flattening. In applanation tonometry the tip of the tonometer is brought into contact with the outer surface of the cornea. Applanation pressure is measured as the surface of the cornea is flattened to a predetermined point. If the eye has a normal internal pressure the applanation pressure required is within a predetermined range. As the internal pressure of the eye increases, the applanation pressure must be increased to obtain the same area of cornea flattening.
One type of applanation tonometer which is believed to give relatively accurate pressure readings is the Goldmann tonometer. To use the Goldmann tonometer a local anesthetic and then fluorescein is applied to the corneal surface. When the eye is exposed to blue light the fluorescein imparts a bright greenish yellow glow to the corneal surface. When the tonometer tip is applied to the outer surface of the eye two bright yellow green semicircular arcs are visualized. As the tip is advanced toward the eye, flattening the cornea, the arcs move across the visual field. Forward movement of the tonometer is continued until the desired end point image is obtained (the inner edge of the upper arc is aligned with the inner edge of the lower arc). The reading on the tonometer is then converted to give the internal pressure of the eye.
While there can be several causes for erroneous readings in use of the Goldmann tonometer, a common source of error is the use of too much or too little fluorescein or dilution of the fluorescein previously applied by excessive tearing. This variation in fluorescein can cause the operator to obtain pressure readings which are either higher or lower than the actual pressure, thus resulting in improper treatment for glaucoma or other causes of elevated pressure.
Besides applanation tonometers, ophthalmic pressures are also determined by using devices incorporating a pressure transducer. These tonometers usually require a cover to prevent moisture from entering the transducer. Additionally, other ophthalmic measuring devices applied to the eye, such as a biometric ruler, include an ultrasonic transducer. These ultrasonic transducers require a cover to retain a film of water against the ultrasonic tip of the device in order to obtain a reading.
Shields have not been effectively used on applanation tonometers as they can interfere with the proper functioning of the device, the obtaining of correct pressure readings, by reducing the visibility of the yellow-green semicircular arcs derived from placing fluorescein on the eye. However, there is a significant concern regarding the transfer of infections from the eye of one patient to a second patient's eye as a result of inadequate disinfecting of the device. This has been resolved to some extent by the application of a disposable, flexible cover over the tip of tonometers utilizing a pressure or an ultrasonic transducer.
Thus, there is a need for a simple, accurate and reproducible method for applying the fluorescein to the eye. Additionally, there is a need for means to prevent transfer of infectious agents from one patient to another while obtaining pressure or other measurements on the eye.